This invention relates to a device for driving a capactive load.
An example of such a capacitive load is a piezoelectric element employed in the printing head of an ink-jet printer. The piezoelectric element in the printing head is supplied with an input pulse signal so that the element is driven for compression and expansion to compress and expand in ink container or chamber containing ink to emit ink droplets in accordance with the input signal. For actuation of the piezoelectric element, a voltage is applied to the element to deform the latter and then the applied voltage is removed or restored to 0 V to take out charges accumulated in the element applied thereto so that the element resumes its original state. The voltage is reapplied to the element and removed therefrom to repeat this cycle. Electrically, this cycle for the compression/expansion drive of the piezoelectric element is accompanied with the charge/discharge operation of the element.
Thus, reference will be made to the case where the capacitive load to be driven is, for example, a piezoelectric element.
A piezoelectric element can be regarded to be equivalent to a capacitor inasmuch as it is a capactive element, and, for the purpose of driving such an element, a charging circuit and a discharging circuit are required for controlling the timing of charging and discharging the element. FIG. 1 shows, by way of example, the basic structure of a driving device of this kind. Referring to FIG. 1, the driving device may include a control circuit 1, a charging switch S.sub.1, a discharging switch S.sub.2, a resistor R.sub.L, a capacitive load or piezoelectric element C.sub.L, and a power source V.sub.cc. In response to the application of a first control signal from the control circuit 1, the charging switch S.sub.1 is turned on, and the piezoelectric element C.sub.L is charged by the power source V.sub.cc. For discharging the piezoelectric element C.sub.L, on the other hand, the control circuit 1 applies a second control signal for turning off the charging switch S.sub.1 and turning on the discharging switch S.sub.2.
FIG. 2 shows the structure of a practical form of the device in which transistors Q.sub.1 and Q.sub.2 connected at their bases with a driver 2 are used as the charging and discharging switches S.sub.1 and S.sub.2, respectively. In the strucure shown in FIG. 2, the NPN transistor Q.sub.1 is turned on to charge the piezoelectric element C.sub.L, while, the PNP transistor Q.sub.2 is turned on (with simultaneous turn-off of the transistor Q.sub.1) to discharge the piezoelectric element C.sub.L. The resistor R.sub.L in FIGS. 1 and 2 serves to determine along with the element C.sub.L the time constant for charging and discharging operations of the element.
When the transistors Q.sub.1 and Q.sub.2 are used to drive the piezoelectric element C.sub.L in the manner shown in FIG. 2, the base current must be continuously supplied to the transistor Q.sub.1 or Q.sub.2 throughout the period of charging or discharging the piezoelectric element C.sub.L driven by the driving signal applied from the driver 2. Consider now, for example, the transistor Q.sub.1. When the transistor Q.sub.1 is turned on the start charging of the load C.sub.L, the emitter potential V.sub.o of the transistor Q.sub.1 rises sharply up to about the level of V.sub.cc due to the turn-on of the transistor Q.sub.1. On the other hand, the charge current flows, attenuating according to the time constant determined by the combination of C.sub.L and R.sub.L. The transistor Q.sub.1 must be kept turned on until this charge current ceases to flow. Although the base current must be continuously supplied to maintain the on-state of the transistor Q.sub.1, this base current is now supplied from the power source V.sub.cc due to the rise of the emitter potential V.sub.o up to about the level of V.sub.cc. Thus, when the transistors are driven with a TTL level signal, a path is necessarily formed through which the driving current flows from the power source V.sub.cc toward the ground. Such an example is shown in FIG. 3 which illustrates the charging section only.
Referring to FIG. 3, there are provided a PNP transistor Q.sub.3 for driving the transistor Q.sub.1 and an NPN transistor Q.sub.4 for driving the transistor Q.sub.3. The transistor Q.sub.4 operates in response to an input signal IN (of TTL level) and acts as a constant-current circuit so as to limit the current supplied from the power source V.sub.cc. The base current I.sub.D of the transistor Q.sub.3 is supplied from the power source V.sub.cc to the ground for turning on the transistor Q.sub.1. The value of this current I.sub.D must be sufficient to permit operation of the transistor Q.sub.1 in its saturation region. Since the transistor Q.sub.4 operates in the active region, and the voltage value of the power source V.sub.cc is usually as large as about 100 V to 300 V. The power loss at the transistor Q.sub.4 becomes a serious problem especially when, for example, the operating frequency is high or the on-duration is long. Even when the transistor Q.sub.1 is replaced by a PNP transistor, a problem similar to that described above still remains unsolved insofar as such a transistor is included in the device.
With a view to obviate the problem pointed out above, a proposal, as, for example, disclosed in Japanese Patent Application Laid-open No. 221517/83, laid-open on Dec. 23, 1983 has been made and already known, in which the transistor Q.sub.1 shown in FIG. 3 is replaced by a thyristor. FIG. 4 shows a structure of a device which employs a thyristor SCR.sub.1 in place of the transistor Q.sub.1. Employment of the thristor SCR.sub.1 is advantageous in that the power consumption can be reduced since the minimum firing current I.sub.gt of the thyristor is small (or usually in the order of 0.1 mA), and the driving current need not be continuously supplied once the thyristor SCR.sub.1 is fired or turned on. In this case, however, it is necessary to turn on the discharging transistor Q.sub.2 after the charging thyristor SCR.sub.1 has completed a charging operation and turn-off of the thyristor has been detected. This is because, otherwise, the thyristor SCR.sub.1 would be continuously turned on. Therefore, when a thyristor is used as the charging switch, the upper limit of the operating frequency is determined by the time constant of the load current and the turn-off time of the thyristor. Also, the device cannot be operated in such a way as to permit starting of discharge midway of charging.